Hydraulic fluids used in automotive hydraulic brake systems must satisfy a variety of requirements. In general, these include chemical and thermal stability, suitable viscosities for the intended use, fluidity over the use-temperature range, low volatility, non-corrosiveness to metals, limited effect on rubber parts and good tolerance for water. Thus, a hydraulic brake fluid to be commercially acceptable is required to meet industry-accepted specifications as well as those established by governmental agencies. Industrial specifications include Society of Automotive Engineers (SAE) specifications such as 7Or1 Artic and 7Or3. Governmental specifications include National Highway Traffic Safety Administration, Department of Transportation, Federal Motor Vehicle Safety Standard 571.116 and 571.116a.
Automotive hydraulic brake fluids used today are most often synthetic glycol-base, water-miscible fluids. Brake fluids suggested in the art to meet the various specification requirements have generally been blends of several components such as lubricants, diluents and one or more oxidation and corrosion inhibitors. Blended hydraulic fluids have contained such lubricants as castor oil, polyoxyalkylene glycols, glycol ricinoleate, and glyceryl ethers of polyoxyalkylene glycols and such diluents as butyl alcohols, amyl alcohols, glycol esters, polyoxyalkylene glycols, ethylene glycol monoalkyl ethers and the like. These blends have not been entirely satisfactory in many instances. Those that are satisfactory with respect to all the requirements are difficult to prepare since a blend component that satisfies one requirement may be deleterious with respect to another requirement. Thus, a blend component that meets a high boiling point requirement frequently does not meet the low temperature viscosity requirement or a blend component that satisfies the low temperature viscosity requirement may adversely affect rubber parts used in hydraulic systems, e.g., cause swelling, softening, and the like.
It is also known that with synthetic glycol-base hydraulic fluids presently used, the water content of the fluid continues to increase with time in service. Normally the water content is 3-4 percent by weight on the average. It can however range as high as 5-6 percent for older cars in more humid areas, and can be as high as 9-10 percent in extreme cases. One undesirable effect of water pickup by a glycol-base hydraulic fluid is the marked increase in the low temperature viscosity of the fluid with its attendant adverse effect on braking performance.